This invention relates to an apparatus and a method for controlling rear wheel braking force of a vehicle to control allocation of front wheel braking force and rear wheel braking force.
When a brake pedal is pressed down, a braking hydraulic pressure (hereinafter called a "master cylinder pressure") generated in a master cylinder is transmitted to wheel cylinders of four wheel to generate braking forces on the individual wheel.
Since, when the amount of pressing down the brake pedal is increased, the braking forces generated on the individual wheels are increased, deceleration of the vehicle increases. When the deceleration of the vehicle increases, a rear wheel load decreased, resulting in a decrease in surface gripping of rear wheels. In such a braking condition that braking (high-G braking) is generated to increase the deceleration of the vehicle, there is a problem in that, when the master cylinder pressure is distributed and transmitted almost equally to the front and wheel cylinders, the rear wheels tend to lock earlier resulting in deteriorated braking stability of the vehicle.
Therefore, a proportioning valve (PCV) has been incorporated in a brake system, which functions to transmit the master cylinder pressure, as is, to the wheel cylinder of the rear wheels when the braking force is small, and reduce increasing rate of hydraulic pressure transmitted to the wheel cylinder of the rear wheels when the master cylinder pressure exceeds a preset value, thereby preventing early locking of the rear wheels.
A prior art braking system will be described with reference to FIGS. 15 to 19. FIG. 15 is a schematic view showing structure of a prior art braking system, FIG. 16 is a schematic view showing hydraulic pressure allocation of the prior art braking system, FIG. 17 and FIG. 18 are schematic sectional views showing condition of a proportioning valve, and FIG. 19 is a schematic view explaining operation of the proportioning valve.
FIG. 15 shows a X-piping braking system commonly used in FF type vehicles, in which numeral 11 indicates a brake pedal. Pressing-down force of the brake pedal 11 is amplified in an assister unit 12 and transmitted to a tandem master cylinder 13.
The master cylinder 13 is provided with two hydraulic pressure generators (not shown) to generate a braking hydraulic pressure according to the amount of pressing down the brake pedal 11. One of the hydraulic pressure generators is connected to a wheel cylinder 15.sub.1 for a left front wheel through a pipe 14, and also to a wheel cylinder 15.sub.4 of a right rear wheel through a pipe 16 branched halfway from the pipe 14 and a PCV 17.sub.2.
The other hydraulic pressure generator is connected to a wheel cylinder 15.sub.2 of a right front wheel through a pipe 18, and also to a wheel cylinder 15.sub.3 of a left rear wheel through a pipe 19 branched halfway from the pipe 18 and a PCV 17.sub.1.
PCVs 17.sub.1 and 17.sub.2 are proportioning valves, which transmit a hydraulic pressure generated by the master cylinder 13, as is, up to a preset value, but when the preset value is exceeded, reduce hydraulic pressure increasing rate to the rear wheels to provide a bent-curved relation in the rear wheel braking force relative to the braking force of the front wheels. This value itself is of a conventional type known in the art, however, its structure to provide the bent-curved relation in hydraulic characteristics will be described with reference to FIGS. 17 to 19.
In FIG. 17, numeral 31 indicates a valve housing. A cylindrical valve chamber 32, with its inner peripheral surface stepwise formed, is formed in the housing 31. The valve chamber 32 comprises a large-diameter cylinder chamber 33 and a small-diameter cylinder chamber 34. A cylindrical valve body 35, movable in the axial direction, is disposed in the cylinder chamber 33, the diameter of the valve body 35 being slightly greater than the diameter of the cylinder chamber 34. A hole h for communicating hydraulic oil is provided from the peripheral surface of the valve body 35 towards the central axis, or from the central axis to the side surface.
Furthermore, a plunger 36 provided in the valve body 35 is slidably inserted into a guide hole 37 provided in the housing 31.
An output port 36 for outputting hydraulic pressure to the wheel cylinder is formed on one side of the cylinder chamber 33, and an input port 39 for inputting hydraulic pressure from the master cylinder 13 is formed on a peripheral surface of the cylinder chamber 34.
A spring 40 is disposed in the cylinder chamber 34, one end of the spring 40 contacts against one side surface of the valve body 35, the valve body 35 is normally pressed towards the output port 38 side by an urging force of the spring 40, and a gap A is formed between the periphery of the valve body 35 and the end portion of the cylinder chamber 34, forming a valve-open condition. That is, an input hydraulic pressure Pi is transmitted as an output hydraulic pressure Po through the gap A and the hole h.
Where So is a pressure-receiving area of the valve body 35 output port side, Si is a pressure-receiving area of the cylinder chamber 34 side, F is an urging force of the spring 40, and Po is an output hydraulic pressure, the valve body 35 is moved to the right or left according to which is greater, "Pi.multidot.Si" or "Po.multidot.So".
As described above, since the gap A is opened by the urging force of the spring 40 in the initial state, the input hydraulic pressure Pi is outputted, as is, as an output hydraulic pressure Po. That is, the output hydraulic pressure increases according to the amount of pressing down the brake pedal 11.
When the output hydraulic pressure Po increases, the value "Po.multidot.So" increases, resulting in Po.multidot.So&gt;Pi.multidot.Pi+F past a setting pressure P1. As a result, the valve body 35 moves towards the cylinder chamber 34 against the urging force of the spring 40, and the gap A is closed by a peripheral edge portion of the valve body 35 as shown in FIG. 18, maintaining the output hydraulic pressure Po. From this condition, when the brake pedal 11 is pressed down even further to increase the input hydraulic pressure Pi resulting in Po.multidot.So&lt;Pi.multidot.Pi+F, the gap A is opened again as shown in FIG. 17, and the output hydraulic pressure Po increases according to the increase in the input hydraulic pressure Pi. As a result, the gap A is closed by the increase in the output hydraulic pressure Po as described above, thereby maintaining the output hydraulic pressure Po. Thus, as shown in FIG. 19, past the setting pressure P1, the output hydraulic pressure Po varies so that the gradient of the output hydraulic pressure Po against the input hydraulic pressure Pi is reduced, and the output hydraulic pressure Po gradually increases past the setting pressure P1.
Magnitude of the setting pressure P1 and the gradient of the output hydraulic pressure Po against the input hydraulic pressure Pi past the setting pressure P1 are determined solely by mechanical constants such as the urging force F of the spring 40, pressure-receiving areas Si and So and the like.
Relationship between a preset braking force allocation and an ideal braking force allocation of the vehicle set by the mechanical factors of PCV17.sub.1 and 17.sub.2 will now be described with reference to FIG. 16. In FIG. 14, A indicates a preset braking force allocation straight line having a bending point showing a preset braking force allocation, and B indicates an ideal braking force allocation curve showing an ideal braking force allocation determined from various parameters of the vehicle.
Here, the ideal braking force allocation means a braking force allocation of front and rear wheels so that a four-wheel simultaneous locking takes place during braking. An intersecting point P11 of the ideal braking force allocation curve B and the dot-bar line of a deceleration of 0.8 G indicates a braking force allocation where the front and rear wheels are simultaneously locked by an abrupt braking with a deceleration of 0.8 G. An intersecting point P12 of the ideal braking force allocation curve B and the dot-bar line of a deceleration of 0.4 G indicates a braking force allocation where the front and rear wheels are simultaneously locked by braking with a deceleration of 0.4 G. Deceleration generated by ordinary braking is 0.2 to 0.3 G.
At points on the straight lines of decelerations of 0.8 G or 0.4 G indicated by dot-bar lines, the total braking forces (sum of the front wheel braking force and the rear wheel braking force) necessary for braking with a deceleration of 0.8 G or 0.4 G are the same.
Furthermore, the two-dot-bar straight lines indicate the braking force of the front or rear wheels for locking the front or rear wheels on a road surface with a friction coefficient .mu. of 0.8 or 0.4. Here, frictional coefficient .mu. of a dry asphalt road surface of fair weather is approximately 0.8.
Thus, point P11 means an ideal braking force allocation of front and rear wheels for simultaneous locking of the front and rear wheels when an abrupt braking at a deceleration of 0.8 G is made on a road surface with .mu.=0.8. Point P12 means an ideal braking force allocation of front and rear wheels for simultaneous locking of the front and rear wheels when a braking at a deceleration of 0.4 G is made on a road surface with .mu.=0.4.
As described above, an ideal braking force allocation curve B for simultaneous locking of front and rear wheels exists, but in practice, it is set so that the braking force of the rear wheels is smaller than the ideal braking force. This is because, if the rear wheels lock earlier than the front wheels, braking stability is deteriorated. That is, the setting braking force is set so that the rear wheel braking force does not exceed the ideal braking force allocation curve B as indicated by straight line A.
When a braking of 0.38 G is made on a road surface with a friction coefficient .mu.=0.4, braking force allocation is made as indicated by an intersecting point P13 of the straight line of a total braking force of 0.38 G and the setting braking force straight line A, the rear wheels will not lock until the rear wheel braking force allocation at the intersecting point P15.
Furthermore, when a braking of 0.38 G is made on a road surface with a friction coefficient .mu.=0.8, the rear wheels will not lock even if the rear wheel braking force is increased up to the rear wheel braking force indicated by the braking force allocation as indicated by an intersecting point P14 of the straight line of a total braking force of 0.38 G and the straight line of .mu.=0.8.
Thus, for braking of the same deceleration, the front wheel braking force can be decreased by Bf according to the road surface condition, and the rear wheel braking force can be increased by Br beyond the ideal braking force allocation.
In other words, as far as the setting braking force straight line A is used, even there is a room in the rear wheel braking force depending on the vehicle traveling and road surface condition, a total braking force is generated by applying a burden to the front wheel braking force to the extent of the room.
In particular, when the vehicle turns, a burden may be applied to the outer front wheel braking force to generate a total braking force even there is a room in the outer rear wheel braking force during a sharp turning. If such an excessive burden is applied to the front wheel braking force, abrasion of the brake pad of the front brake unit may increase and heat evolution increase, resulting in a fade phenomenon where the friction coefficient of the brake pad considerably decreases and a vapor lock due to a temperature increase of brake fluid. Furthermore, this leads to a nose diving at braking, deteriorating the braking stability.
As the prior art based on the similar concept, there have been examples such as Japanese Patent Publication Laid-open 1-257652/1989 (DE3742173, FR2624462, GB2213543), Japanese Patent Publication Laid-open 3-125657 (GB2236156, DE3931858), and Japanese Patent Publication Laid-open 3-208760 (DE4029332, GB2238092, FR2654401).
In these prior art examples, the action of a proportioning valve is invalidated by an electromagnetic valve in the normal condition to increase the braking force allocation to the rear wheels and lighten the burden to the front wheel brake unit, and the electromagnetic valve is operated to effect the action of the proportioning valve only when an antilocking unit malfunctions.
However, these prior art examples do not consider braking force allocation during turning of the vehicle, the proportioning valve is operative only when the antilocking unit malfunctions, and the functions of the proportioning valve cannot be effectively utilized.
With a view to eliminate the above prior art problems, it is a primary object of the present invention to provide a rear wheel braking force control apparatus which, during turning of the vehicle, increases the braking force to the outer rear wheel, which has a room in the braking force, more than the braking force allocation to the inner rear wheel, thereby improving braking stability and braking characteristics during turning of the vehicle.